1. Field of the Invention
The present invention relates to a measuring device for a vertical seismic profile within a well-bore.
2. Description of the Prior Art
Measurements taken in a well-bore usually fall into two main classes. The first class is known as ultrasonic measurement. This type of measuring operation is performed by means of a tool which is lowered into the well-bore at the end of a cable. The tool comprises one or a number of transmitters and one or a number of receivers in addition to the means required for turning-on the transmitters at suitable moments, and means for transmitting the signals received by the receivers to the ground surface for the purpose of recording and processing said signals.
Ultrasonic measurement makes use of transmitters which are capable of transmitting high-frequency sound signals within the range of a few thousand hertz to several tens of thousands of hertz into the fluid which fills the well. The sound signals propagate within the fluid in the form of compression waves in which the associated physical parameters are essentially the pressure and velocity of particles.
It should be noted that waves of different types may be generated as the measuring tool comes into contact with heterogeneous formations around the tool and especially as this latter comes into contact with the wall of the bore-hole. However, ultrasonic measurements are usually performed with a view to ensuring that waves other than compression waves are reduced or highly attenuated and that errors are consequently not liable to occur when processing compression waves of interest.
In the majority of instances, ultrasonic measurements are performed during displacement of the tool, that is to say when the tool is moving within the well-bore. The relatively low speed of the tool within the bore (a few meters per minute) does not give rise to parasitic noise in the receivers.
The receivers employed (also known as transducers) are hydrophones which in fact consist of vibrational pressure sensors or so-called pickups. The waves received by the pickups receive the compression waves defined earlier and possibly any parasitic waves derived from the compression waves and generated by heterogeneous formations. The time-duration of each useful signal received by the pickup after each transmission is of the order of a few milliseconds, taking into account the frequency of the signals transmitted.
It is very important to note that measurement of a compression wave received by the pickup, which is a measurement of vibrational pressure, makes it possible to obtain only a scalar quantity, that is to say without orientation in space.
Many different types of tools for ultrasonic measurement are in existence at the present time. These tools secure freedom from waves which propagate within the actual mass of the tool and make it possible to form acoustic paths having cylindrical symmetry about the axis of the well-bore.
The second type of measurement relates to a vertical seismic profile, an oblique seismic profile or else a seismic well-profile, this second type of measurement being completely different from the first type recalled earlier by way of reference.
In fact, when prospecting for petroleum, for example, it is necessary to determine the nature and characteristics of subsurface strata which surround the bore-hole. To this end, seismic energy is generated at the ground surface and not within the bore-hole by means of a detonation (explosive charge), a suitable impact (caused by a falling weight), or else a vibration in contact with the ground surface. This seismic energy produces waves which propagate within the subsurface strata and which are received by geophones and no longer by pickups. The geophones are designed in the form of "velocimeters" which are sensitive to vibrational velocity and placed either on the ground surface, in which case they receive the waves reflected and/or refracted by the different subsurface strata, or within the drill-hole but in this case they must imperatively be in direct contact with the wall of the drill-hole in contrast to ultrasonic measurement in which pickups have to be within the fluid which fills the bore-hole.
In all cases, geophones or velocimeters are sensitive to vibrational velocity and must permit determination of all types of seismic waves generated by the energy emitted at the surface. In fact, direct mechanical coupling between the emitting means and the ground surface gives rise to all types of waves since the seismic energy emitted necessarily encounters heterogeneous formations in the subsurface under exploration. In consequence, it can no longer be considered sufficient to receive one particular wave among others on one or a number of geophones or to obtain a scalar quantity. On the contrary, the geophones must necessarily receive all the waves including compression or transverse waves and in all directions in which they propagate in order that all possible directions of vibrational velocities may accordingly be distinguished at the time of processing.
Taking into account the transmission means employed and the object to be achieved, frequencies are transmitted within the range of a few hertz to a few hundreds of hertz (for example within the range of 5 to 500 Hz) and the time-duration of the useful signal is of the order of a few seconds.
Finally, in contrast to ultrasonic measurement, it is worthy of note that, in the second type of measurement considered and during upward travel of the tool, the measurements are performed while the geophone or geophones remain stationary with respect to the bore-hole wall and are applied against the wall in order to ensure satisfactory coupling.
Subsequent processing of the signals received at the geophones makes it possible to obtain useful information on the subsurface strata traversed by the waves from the ground surface to the geophones.
A number of different techniques may be employed. It is possible to place geophones at intervals (which may or may not be uniform) along the bore-hole and to process the data on the different geophones. It is also possible to employ a tool which comprises a geophone and which is stopped at each appropriate level of the bore-hole during each measurement.
In the second type of measurement, the major difficulty arises from the fact that no method has yet been found for obtaining satisfactory coupling of the geophone with the wall of the bore-hole.
One attempt has been made to overcome the difficulty and has been described in French patent No 1,169,871. The device of the prior art comprises a tool having a cylindrical element which serves to maintain a geophone assembly at the desired height within the well-bore and prevents said geophone assembly from rotating with respect to the tool by means of a key connection. The geophone assembly comprises two arcuate blade-springs disposed symmetrically on each side of the tool which is centered within the well-bore. Said blade-springs are connected to each other by means of pivots mounted on an intermediate element which is connected to the tool body. One of the blade-springs is applied against the wall of the bore-hole while the other blade-spring is adapted to carry substantially at the vertex of its arch a shoe which is pivotally mounted at the center of the spring. The outer face of said shoe is in contact with the wall opposite to that on which the first spring is applied. A geophone is then fixed on the inner face of the shoe and is connected by means of a conductor cable t the amplification equipment located within the tool body.
A tool of this type constitutes a substantial improvement upon the technique of geophones placed on the ground surface or distributed along the wall of the bore-hole. But it fails to provide a complete solution to the problem of direct coupling with the wall of the bore-hole.
There is in fact no direct contact of the geophone with the wall of the bore-hole since it is mounted on the internal face of the shoe. Although the shoe is perfectly rigid and the tension of the springs prevents the shoe from sliding along the wall of the bore-hole during a measurement operation, it is nevertheless a fact that the parasitic waves which propagate within the tool body, which are produced by the transmitted energy and which also propagate within the blade-springs and the shoe are received by the geophone. This arises from the fact that the tool, the shoe and to a lesser degree the blade-springs are rigid.
A further disadvantage is that, in the case of a given measurement, the geophone is oriented in only one direction and is therefore capable of detecting only one type of wave.
A further drawback which is probably the most serious is that the blade-spring which supports the geophone has only one degree of freedom as determined in the direction of the arch of the spring. The structure of the spring and the application stresses developed do not permit flexibility of the spring in all directions. In consequence, there is no effective decoupling between the mass of the tool, the mass of the geophone and of the associated shoe. The mass of the tool is of very considerable value with respect to that of the geophone assembly and introduces a parasitic effect which is greater as the connection of the tool body with said geophone assembly has greater rigidity.
Another point worthy of note is that, by reason of the rigid connection between the mass of the tool and the geophone, the geophone whose mass is of low value with respect to that of the tool cannot usefully vibrate at the time of reception of high frequencies of the transmission spectrum. These high frequencies are thus eliminated and the general result thereby achieved is the same as if the tool body and the relative flexibilities of the bore-hole walls constituted a filter for these high frequencies.
The present invention which relates to the second type of measurement offers a satisfactory solution to the problem thus presented and makes it possible to overcome the disadvantages recalled in the foregoing.